First of all, most of us underestimate the number of calories we consume in a day, underestimate the caloric content of foods, and overestimate the number of calories we burn during expenditure. It takes a deficit of 3500 calories to lose a pound of fat and vice versa for weight gain. So look at it this way, if you have a caloric deficit of 500 calories/day in one week you will lose 1 pound (which is recommended).
How do we figure this? Follow the link below for great information on caloric balance (courtesy of the CDC).
Also, here are a few useful websites and or apps that can help you track your calories in and out:
http://caloriecount.about.com/ (they also have an app)
And don't forget, there is also the old reliable pen and paper for recording. You can then use the above sites to figure out calorie content and expenditure.